1. Field of the Invention
The invention relates to a cut-off level setting device, a method of setting a cut-off level, and a display apparatus with a cathode ray tube and a cut-off level setting device.
In a cathode ray tube, an electron gun comprises a cathode heated by a heater filament, a first grid, a second grid, and further grids for focussing an electron beam emitted by the cathode. In a color cathode ray tube, three electron guns may be present, each of which generates an electron beam which is deflected to hit the corresponding one of three phosphors which emit light of different colors. The beam current (the number of electrons in the electron beam, also referred to as cathode current) is largely dependent on a voltage difference between the cathode and the first grid. However, the second grid voltage also has an influence on the beam current, but this influence is substantially less pronounced. The beam current may be modulated by varying the cathode voltage in dependence on the video information to be displayed. At a voltage difference of zero volts between the cathode and the first grid, the maximal beam current flows. When the voltage on the cathode becomes more positive with respect to the first grid, the beam current decreases. At a certain value of the cathode voltage the beam current becomes exactly zero. The corresponding voltage difference between the cathode and the first grid is generally referred to as the cut-off voltage or black level voltage. The corresponding voltage level on the cathode is referred to as the cut-off drive voltage level. Varying the second grid voltage changes this cut-off voltage. Consequently, the second grid voltage also determines the maximum beam current which can be generated by the electron gun.
2. Description of the Related Art
International Patent Application WO-A-97/28525, corresponding to U.S. Pat. No. 5,892,337, discloses a black level or cut-off level setting method wherein two control voltage levels are supplied to a cathode in a predetermined ratio to generate two cathode currents. The two cathode currents are measured to compare each one of the cathode currents with an associated one of two reference values having a predetermined ratio. The ratio of the reference values has a predetermined relation to the ratio of the two control voltages. 0Subtracting the associated reference value from each of the cathode currents generates two error voltages. One of the error voltages is integrated to obtain a black level setting value, the other error voltage is integrated to obtain a gain setting value. The black level setting value controls the DC-level of the control signal to be very near to the cut-off point of the cathode when a black video signal has to be displayed. The error voltages are controlled in a control loop in an iterative way to become zero after some time, such that the desired ratios are reached.